This invention relates to heat transfer systems, and more specifically, to a recuperative and conductive heat transfer system that is operative to effect therewith the heating of a xe2x80x9cworking fluidxe2x80x9d by means of the transfer of heat from hot regenerative solids to the xe2x80x9cworking fluidxe2x80x9d. The term xe2x80x9cworking fluidxe2x80x9d as employed herein is intended to refer to the xe2x80x9cworking fluidxe2x80x9d of a thermodynamic cycle, e.g., steam or ammonia, as well as to a process feedstock. The source of heat by means of which the hot regenerative solids themselves become heated may take many forms with the most prevalent of those commonly being that of an internal heat source, e.g., being that of the hot gases, which are produced as the result of the combustion of fuel and air in some type of combustion chamber. However, this source of heat could also be in the form of an external heat source, e.g., be in the form of the hot gas exhaust from a turbine or other similar piece of equipment, or could be in the form of a hot process stream, which is produced as a consequence of some kind of chemical reaction.
With further reference to the matter of internal heat sources, furnaces for firing fossil fuels have long been employed as a device for generating controlled heat with the objective of doing useful work. To this end, the work application might he in the form of direct work, as with rotary kilns, or might be in the form of indirect work, as with steam generators for industrial or marine use or for the generation of electric power. A further differentiation, insofar as such furnaces is concerned, is whether the furnace enclosure is cooled, such as with waterwalls, or uncooled, such as with a refractory lining.
It is believed that such furnaces developed originally from a need to fire pottery, around 4000 B. C., and a need to smelt copper, in or about 3000 B. C. Hastening and improving combustion by the use of bellows to blow air into the furnace is believed to have occurred in about 2000 B. C.
Closely associated with such furnaces is the corresponding steam boiler. Such boilers appear to be of Greek and Roman origin and were employed for household services. The Pompeiian water boiler, incorporating the water-tube principle, is one of the earliest recorded instances, i.e., in approximately 130 B. C., of boilers doing mechanical work. To this end, the Pompeiian water boiler sent steam to Hero""s engine, a hollow sphere mounted and revolving on trunnions, one of which permitted the passage of steam, which was exhausted through two right-angled nozzles that caused the sphere to rotate. This is considered by most people to have been the world""s first reaction turbine.
For virtually the next 1600 years, furnaces in general and waterwall furnaces in particular were essentially a neglected technology. This can be partly ascribed to the fact that steam as a working.fluid had no application until the invention of the first commercially successful steam engine by Thomas Savery in 1698. In 1705, Newcomen""s engine followed and by 1711, this engine was in general use for pumping water out of coal mines. Self-regulating steam valves are believed to have first come into existence in 1713.
Many varieties of firetube boilers were invented in the second half of the 18th century, culminating with the so-called Scotch marine boiler. As the name firetube boiler would imply, in the firetube boiler the tubes may be considered to be a component part of the furnace, with the combustion process-taking place within the tube bundles. However at the time, such units were limited, because of available steel-plate thicknesses, to operating pressures of about 150 psig. This was then followed by the development of the modem water-tube furnace for steam generation at higher pressures and in larger sizes than available with firetube boilers. Today, such modern water-tube furnaces for steam generation encompass all of the following: central-station steam generators, industrial boilers, fluidized-bed boilers, and marine boilers.
Of all of these various types of boilers, if it were necessary to classify the recuperative and conductive heat transfer system to which the present application is directed into one of these types of boilers, the recuperative and conductive heat transfer system to which the present application is directed, to the extent that an internal heat source is employed in connection with such a recuperative and conductive heat transfer system, would probably be considered to be more akin to a fluidized-bed boiler than to any of the aforementioned other various types of boilers. As such, the focus of the discussion hereinafter insofar as the prior art is concerned will thus be directed primarily to the fluidized-bed boiler type. To this end, fluidized-bed reactors have been used for decades in non-combustion reactions in which the thorough mixing and intimate contact of the reactants in a fluidized bed result in high product yield with improved economy of time and energy. Although other methods of burning solid fuels can generate energy with very high efficiency, fluidized-bed combustion can burn solid fuel efficiently at a temperature low enough to avoid many of the problems of combustion in other modes.
To those in the industry, it is well known that the word xe2x80x9cfluidizedxe2x80x9d as employed in the term xe2x80x9cfluidized-bed boilerxe2x80x9d refers to the condition in which solid materials are given free-flowing fluid-like behavior. Namely, as a gas-is passed through a bed of solid particles, the flow of gas produces forces that tend to separate the particles from one another. At low gas flows, the particles remain in contact with other solids and tend to resist movement. This condition is commonly referred to as a fixed bed. On the other hand, as the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed then becomes fluidized, that is, the gas cushion between the solids allows the particles to move freely, giving the bed a liquid-like characteristic.
The state of fluidization in a fluid-bed-boiler combustor depends mainly on the bed-particle diameter and fluidizing velocity. As such, there are essentially two basic fluid-bed combustion systems, each operating in a different state of fluidization. One of these two basic fluid-bed combustion systems is characterized by the fact that at relatively low velocities and with coarse bed-particle sizes, the fluid bed is dense, with a uniform solids concentration, and has a well-defined surface. This system is most commonly referred to by those in the industry as a bubbling fluid bed, because the air in excess of that required to fluidize the bed passes through the bed in the form of bubbles. The bubbling fluid bed is further characterized by modest bed solids mixing rates, and relatively low solids entrainment in the flue gas. While little recycle of the entrained material to the bed is needed to maintain bed inventory, substantial recycle rates may be used to enhance performance.
The other of these two basic fluid-bed combustion systems is characterized by the fact that at higher velocities and with finer bed-particle size, the fluid bed surface becomes diffuse as solids entrainment increases, such that there is no longer a defined bed surface. Moreover, recycle of entrained material to the bed at high rates is required in order to maintain bed inventory. The bulk density of the bed decreases with increasing height in the combustor. A fluidized-bed with these characteristics is most commonly referred to those by those in the industry as a circulating fluid bed because of the high rate of material circulating from the combustor to the particle recycle system and back to the combustor. The circulating fluid bed is further characterized by very high solids-mixing rates.
There are numerous examples to be found in the prior art of various forms of fluidized bed combustion systems, which have been devised over the course of time. Going back to as early as the late 1950""s, an early example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 2,818,049 entitled xe2x80x9cMethod of Heatingxe2x80x9d, which issued on Dec. 31, 1957. In accordance with the teachings of U.S. Pat. No. 2,818,049, there is provided a method of transferring heat from a burning fluid involving the use of a fluidized pseudo-liquid bed of discrete material, which is an oxidizing catalyst and is continuously circulated by gravity through a predetermined path that includes an upflow column and a downflow column. Continuing, the subject method includes the steps of maintaining the bed in a fluidized pseudo-liquid state and the density of the upflow column substantially lower than the density of the downflow column by generating combustion gases in the upflow column through the introduction and burning of fuel therein, flowing the combustion gases upwardly through the upflow column, disengaging a portion of the combustion gases from the upflow column at the upper end of the upflow column, passing a fluid in indirect heat exchange relation with the bed at a location in the upflow column above the introduction and burning of fuel therein to impart heat thereto and maintaining the rate of circulation of the bed such that the temperature of the bed and accordingly of the entraining gases immediately downstream of the aforementioned location is substantially less than that immediately upstream of the aforementioned location.
A second example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 2,983,259 entitled xe2x80x9cMethod and Aparatus of Steam Generationxe2x80x9d, which issued on May 9, 1961. In accordance with the teachings of U.S. Pat. No. 2,983,259, a lowermost heat exchange zone is provided. The material in this lowermost heat exchange zone is preferably made up at least in part of an active oxidation catalyst so as to give this zone a sufficiently high catalytic activity that a fuel-air mixture may be introduced directly thereinto and effectively and efficiently oxidized therein, liberating heat and accordingly producing a hot stream of gases that pass upwardly through the material with a portion of this heat being absorbed in this zone as well as the heat exchange zones located above this zone. In order to have efficient and complete combustion or oxidation within a fluidized bed of practical height and with the combustion supporting gas being preheated to a reasonable degree it is essential that an active oxidation catalyst be employed so that the material has sufficient catalytic activity to effect complete oxidation of the fuel and it is further essential when the heat content of the fuel is at all substantial that means be provided in contact with the material of this bed for absorbing substantial quantities of heat from the fluidized material in order that the temperature of the material will not rise above the deactivation temperature of the catalyst employed, i.e., the temperature above which the catalyst is permanently damaged so that it loses all or a vast majority of its catalytic activity.
A third example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 2,997,031 entitled xe2x80x9cMethod of Heating and Generating Steamxe2x80x9d, which issued on Aug. 22, 1961. In accordance with the teachings of U.S. Pat. No. 2,997,031, a fuel-air mixture is passed over a body of catalytic oxidizing material, which may take the form of a very thin layer of particles, with this relatively small quantity of material having a very high catalytic activity with a low activation temperature and accordingly being a relatively expensive catalyst. The fuel-air mixture passing over this material is catalytically oxidized and the hot combustion gases thus produced are passed through the bed of material within which the conduit is immersed thereby raising the temperature of this material. The fuel and air is regulated so as to raise the temperature of this bed of material to the point where a fuel and air mixture introduced into the bed will be completely oxidized. Thereafter fuel and air are supplied to this bed and oxidized therewithin with little or no fuel then being passed over and in contact with the high activity catalyst.
A fourth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 3,101,697 entitled xe2x80x9cSteam Generationxe2x80x9d, which issued on Aug. 27, 1963. In accordance with the teachings of U.S. Pat. No. 3,101,697, an oxidation catalyst is employed immediately upstream of a bed of material which is required to be heated to a much higher temperature than the oxidation catalyst before a fuel-air mixture will be oxidized or burned within the bed of material. A housing is provided within which is disposed a bed of discrete material. This bed of material is supported upon a plurality of horizontally disposed elongated members extending across the housing and disposed in generally parallel spaced relation such that the material cannot pass downward past these members but fluidizing gas may pass upwardly therethrough. These members are coated or impregnated with an active oxidation catalyst such that the activation temperature of the catalyst is substantially below the minimum bed temperature which is required to oxidize a fuel-air mixture. Means are provided to force air upwardly through the housing over the elongated members and through the bed of material to fluidize this material with an air heater being employed to heat the air sufficiently to raise the temperature of the catalyst to its activation temperature. Below the elongated members are a plurality of fuel distribution conduits and immediately above these members and in the lower portion of the bed there is another group of fuel distribution conduits. In operation, the fuel distribution conduits below the elongated members are first used to inject fuel into the housing and this fuel mixes with the air and is oxidized by the catalyst with the heat thus developed heating the bed of material or a portion of the bed to its required minimum temperature. Thereafter fuel is introduced into the fuel distribution conduits immediately above the elongated members and the supply of fuel below these members is terminated. In lieu of providing separate fuel distribution conduits below the elongated members, which support the bed, these members may be hollow with downwardly facing openings provided therein so that the members themselves form distribution conduits to which fuel may be supplied.
A fifth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 3,115,925 entitled xe2x80x9cMethod of Burning Fuelxe2x80x9d, which issued on Dec. 31,1963. In accordance with the teachings of U.S. Pat. No. 3,115,925, a start-up procedure is provided wherein the ignition temperature of the fluidized bed is greatly lowered. To this end, a catalyzing solution of a metal salt is sprayed or otherwise introduced onto the bed of particulate material, and thereafter the bed is preheated until ignition temperature has been reached. The dried residue of the salt remaining on the surface of the particles in the fluidized bed catalyze the ignition
of the natural gas and the air at a much lower temperature than the 1150 degrees F., which would otherwise be the ignition point.
A sixth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 3,119,378 entitled xe2x80x9cSteam Generationxe2x80x9d, which issued on Jan. 28, 1964. In accordance with the teachings of U.S. Pat. No. 3,119,378, a method of heating a fluid is provided comprising flowing upwardly a fluidized bed of discrete oxidation catalyst, which has an activation and a deactivation temperature, with the deactivation temperature being well below flame temperature, and a fuel-air mixture that is sufficiently rich in fuel so that it is outside the range of inflammability effecting catalytic oxidation of the fuel within the bed to the extent permitted by the air contained in the mixture while maintaining the temperature of the catalyst below the deactivating temperature, passing the remainder of the fuel and other effluent from the-bed upwardly through another fluidized bed of discrete inert material that is unaffected by flame combustion, thereby heating the material substantially to the temperature of the effluent and oxidizing sufficient fuel in the bed of catalyst to raise the temperature of the other bed to a sufficiently high value so as to oxidize a fuel-air mixture therein while maintaining the catalyst below its deactivation temperature, introducing sufficient air into this other bed to support combustion of this remaining portion of the fuel, effecting oxidation of the remaining fuel portion in this other bed, and imparting heat from the bed to a fluid by passing a fluid in indirect heat exchange relation with the beds.
A seventh example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 4,325,32 entitled xe2x80x9cHybrid Fluidized Bed Combustorxe2x80x9d, which issued on Apr. 20, 1982. In accordance with the teachings of U.S. Pat. No. 4,325,327, a first atmospheric bubbling fluidized bed furnace is combined with a second, turbulent circulating fluidized bed furnace to produce heat efficiently from crushed solid fuel. The bed of the second furnace receives the smaller sizes of crushed solid fuel, unreacted limestone from the first bed, and elutriated solids extracted from the flue gases of the first bed. The two-stage combustor of crushed solid fuel is alleged to provide a system with an efficiency greater than that available through the use of a single furnace of a fluidized bed.
An eighth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 4,335,662 entitled xe2x80x9cSolid Fuel Feed System For A Fluidized Bedxe2x80x9d, which issued on Jun. 22, 1982. In accordance with the teachings of U.S. Pat. No. 4,335,662, a fluidized bed for the combustion of coal, with limestone, is replenished with crushed coal from a system discharging the coal laterally from a station below the surface level of the bed. A compartment, or feed box, is mounted at one side of the bed and its interior is separated from the bed by a weir plate beneath which the coal flows laterally into the bed, while bed material is received into the compartment above the plate to maintain a predetermined minimum level of material in the compartment.
A ninth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 4,360,339 entitled xe2x80x9cFluidized Boilerxe2x80x9d, which issued on Nov. 23, 1982. In accordance with the teachings of U.S. Pat. No. 4,360,339, there is provided a fluidized bed cell having a static ignition bed of inert heat storage particles disposed immediately beneath and adjacent to a fluidizing region wherein fuel particles are combusted, characterized in that the heat storage particles are generally spherical in shape, each particle having a plurality of protuberances extending outwardly from the surface of the particle a preselected length thereby maintaining a minimum spacing equal to the preselected length of the protuberances, between neighboring spherical particles within the static ignition bed, thereby ensuring that sufficient void space exists within the static ignition bed for the fluidizing air to flow upward through the static ignition bed into the fluidizing region without an excessive pressure drop and for the fuel particles to laterally penetrate the static ignition bed.
A tenth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 4,445,844 entitled xe2x80x9cLiquid Fuel And Air Feed Apparatus For Fluidized Bed Boilerxe2x80x9d, which issued on May 1, 1984. In accordance with the teachings of U.S. Pat. No. 4,445,844, a fluidized bed furnace is provided in which liquid fuel can be burned. Injectors extend up through an imperforate bed plate which properly mix the oil or other liquid fuel with the fluidizing air, causing evaporation of the oil. This mixture is passed through restricted openings as the mixture enters the fluidized bed, thus resulting in high velocity flow and fairly even fuel and combustion distribution throughout the cross-section of the fluidized bed.
An eleventh example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 4.633,818 entitled xe2x80x9cMobile Coal-Fired Fluidized Bed Power Unitxe2x80x9d, which issued on Jan. 6, 1987. In accordance with the teachings of U.S. Pat. No. 4,633,818, a mobile coal-fired fluidized bed furnace system is provided for generating steam to power a locomotive. Coal is combusted within the fluidized bed furnace chamber in the fluidizing air to produce a hot flue gas, which passes from the furnace chamber through a boiler bank and an economizer. The steam generated in the boiler bank and the walls of the furnace chamber is collected in a steam drum and is passed therefrom through an in-bed superheater, and thence to the power generating means to produce the power, which drives the locomotive.
A twelfth example thereof, by way of exemplification and not limitation in this regard, is that which forms the subject matter of U.S. Pat. No. 5,401,130 entitled xe2x80x9cInternal Circulation Fluidized Bed (ICFB) Combustion System And Method Of Operation Thereofxe2x80x9d, which issued on Mar. 28, 1995. In accordance with the teachings of U.S. Pat. No. 5,401,130, there is provided a fluidized bed combustion system particularly suited for use to effect the incineration, i.e., combustion, therewith of wood waste/sludge mixtures that have high moisture and ash content, which makes them difficult to burn. The fluidized bed combustion system includes a fluidized bed combustor embodying a fluidized bed composed of bed solids. Air is injected into the fluidized bed through an air distributor to establish a first controlled fluidizing velocity zone and a second controlled fluidizing velocity zone therewithin. Material is introduced into the fluidized bed combustor above the second controlled fluidizing velocity zone, whereupon the bed solids rain down upon the material, which is so introduced, and effect a covering thereof. The material is then dried, and thereafter combusted. Inerts/tramp materials/clinkers, as well as large diameter solids, entrained with the material are segregated therefrom, and then are removed from the fluidized bed combustor.
By way of exemplification and not limitation, the following are several other examples of prior art forms of fluidized bed units. The first of these is set forth in the paper entitled xe2x80x9cAn Introduction To The Solids Circulation Boilerxe2x80x9d by J. G. Ballantyne, which was presented at the Coaltech ""87 Conference that was held June 9-11, 1987 in London, England. In accordance with the mode of operation of the subject boiler as set forth in the aforereferenced paper, combustion takes place in a dense bubbling fluidized bed, arranged in 3 zones so that particle residence time is maximized. The bed is made up of sized, fused alumina beads plus fuel ash and limestone, and the main fluidizing velocity is selected such that only fine particles of ash and stone leave the bed with the flue gases. Because of the modest grit loadings and gas velocities, the flue gases can be passed through convective surface before being discharged to a multi-cyclone. This multi-cyclone, handling cooled gases, is of mild steel construction, and returns coarse particles of limestone and unburnt material to the bed for reuse. From the bed, a controlled quantity of material is continuously extracted by a non-mechanical valve, and cooled in a water-cooled channel integral with the boiler structure. The transport air required for carrying this material is used for secondary combustion purposes.
The originator of the boiler, which forms the subject matter of the above-entitled paper, is alleged to have been a W. B. Johnson. This is believed to be the same W. B. Johnson, who is the named inventor on U.S. Pat. No. 4,539,939 entitled xe2x80x9cFluidized Bed Combustion Apparatus And Methodxe2x80x9d, which issued on Sep. 10, 1985. In accordance with the teachings of W. B. Johnson""s U.S. Pat. No. 4,539,939, a plurality of relatively dense bead-like particles of inert solid material are maintained dispersed throughout the fluidized combustion bed for circulation through heat exchange means separated from the combustion bed, and are returned to the fluidized bed along with other bed constituents. Fine limestone particles may also be introduced into the combustion bed along with fresh fuel particles. The circulating bed constituents are discharged from an arched heat exchange outlet to direct the returning bed constituents in a generally horizontal direction directly over the combustion bed for generating increased circulation in the bed. In addition, the inlet for introduction of fresh fuel and fine limestone is located just below the arched discharge channel to enhance horizontal discharge velocity. A portion of the combustion chamber, generally opposite the arched discharge channel, is provided with a sloped wall segment to.further enhance circulation within the bed.
Before concluding with this discussion of prior art forms of fluidized bed units, it is believed that it is important that there be attention focused on several aspects of such prior art forms of fluidized bed units, particularly as regards the mode of operation and the nature of the construction of such prior art forms of fluidized bed units. To this end, it should be pointed out for instance that in accordance with the mode of operation and the nature of the construction of prior art forms of fluidized bed units and in particular, prior art forms of large circulating fluidized bed units, typically in such prior art forms of large circulating fluidized bed units the fine solid fuel ash/sorbent particles are separated from the flue gas before these fine solid fuel ash/sorbent particles are caused to flow to and through a fluid bed heat exchanger. As such, there is, therefore, no attempt made to classify the type of solid particles, which are caused to flow to and through the fluid bed heat exchanger. To this end, in accordance with such a mode of operation, the solid particles, which are caused to flow to and through the fluid bed heat exchanger, consist entirely of a mixture of all of the ash, which has been produced as a consequence of the combustion of the solid fuel in the presence of air within the combustor of such a prior art form of a large circulating fluidized bed unit.
In addition, attention, it is believed, should also be directed to the fact that in accordance with the mode of operation and the nature of the construction, in particular, of prior art forms of large circulating fluidized bed units, when fluid bed ash coolers are employed in such prior art forms of large circulating fluidized bed units such fluid bed ash coolers typically are used to cool the ash, which has been produced as a consequence of the combustion of the solid fuel in the presence of air within the combustor of such a prior art form of large circulating fluidized bed unit, as such ash is made to leave said prior art form of large circulating fluidized bed unit. Such a fluid bed ash cooler, it is recognized, may operate to effect a separation of large ash particles from the fines entrained therewith, before such separated fines are made to return to said large circulating fluidized bed unit. However, here again as was discussed previously in the preceding paragraph there is no attempt made in the case of the fluid bed ash cooler either to classify the type of solid particles, which collectively comprise the ash, which has been produced as a consequence of the combustion of the solid fuel in the presence of air in the combustor of said prior art form of large circulating fluidized bed unit. Namely, as was discussed in the preceding paragraph, the solid particles, which are separated by operation of such fluid bed ash coolers, consists entirely of a mixture of all of the ash that has been produced as a consequence of the combustion of the solid fuel in the presence of air in the combustor of said prior art form of large circulating fluidized bed units.
Further in this regard, attention is directed to the fact that in accordance with the teachings of U.S. Pat. No. 4,539,939 to which reference has previously been had herein, bed materials embodying bauxite are withdrawn from the bubbling bed. However, there is no disclosure to be found in said teachings of said U.S. Pat. No 4,539,939 of any attempt being made to separate any residual ash or fuel from the bed material embodying bauxite before such bed material embodying bauxite is caused to flow to the heat exchanger.
Therefore, by way of summary in this regard, it has thus been the customary practice insofar as prior art forms of fluidized bed units, and in particular, prior art forms of large circulating fluidized bed units are concerned, that no attempt is made in accordance with the mode of operation and the nature of the construction of such prior art forms of fluidized bed units to effect in the operation thereof a classification/separation between the various types of solid particles, before they are made to return to a fluid bed heat exchanger. Most importantly, it is respectfully submitted that no such attempt at classification/separation between the various types of solid particles is either disclosed or even suggested by the prior art in connection with a fluid bed heat exchanger and in particular where such a fluid bed heat exchanger comprises a counter flow heat transfer system. More specifically, it is respectfully submitted that there is no teaching or even suggestion to be found in any of the prior art documents to which reference has been had hereinbefore of effecting a classification/separation between the types of solid particles, which collectively comprise the ash that has been produced as a consequence of the combustion of the solid fuel in the presence of air in the combustor of prior art forms of fluidized bed units, either before or after such solid particles, are made to flow through a counter flow heat transfer system.
Although the fluidized-bed boilers constructed in accordance with the teachings of the various U.S. patents to which reference has been had hereinbefore, as well as the fluidized-bed boiler that forms the subject matter of the aforereferenced paper that was presented at the Coaltech ""87 Conference have been alleged to have been demonstrated to be operative for the purpose for which they have been designed, there has been evidenced in the prior art a need for such fluidized-bed boilers to be further improved. More specifically, there has been evidenced in the prior art a need for a low cost heat transfer system embodying a design, which is predicated on a novel approach, and which is characterized by its solids enhanced heat transfer. To this end, a fundamental characteristic, which is not surprising in view of the term employed to refer thereto, i.e., xe2x80x9cfluidized-bed boilerxe2x80x9d, of all such fluidized-bed boilers constructed in accordance with the teachings of the various U.S. patents to which reference has been hereinbefore, as well as the fluidizing-bed boiler that forms the subject matter of the aforereferenced paper that was presented at the Coaltech ""87 Conference, is the need for the utilization therein of fluidizing air in order to effect the operation of the fluidized-bed boiler, regardless of whether the fluidized-bed boiler is designed to employ a bubbling bed type mode of operation or a circulating fluidized bed type mode of operation. Namely, regardless of whether a bubbling type mode of operation is employed or whether a circulating fluidized bed type mode of operation is employed, there still nevertheless exists a requirement that fluidizing air be utilized for some purpose if the desired mode of operation is to be accomplished effectively. Such fluidizing air, irrespective of whether a bubbling bed type mode of operation is being employed or whether a circulating fluidized bed type mode of operation is being employed, is designed to be injected at a preselected velocity, the selection of which is determined principally by the fact of whether the particular fluidized-bed boiler is intended to be operated in a bubbling bed type mode or in a circulating fluidized bed type mode, whereby such fluidizing air is caused to flow through a bed comprised of particles of materials, the nature of which may take many forms, e.g., fuel particles, limestone particles, inert particles, etc. As such, because of the need heretofore for the use of such fluidizing air in prior art forms of fluidized-bed boilers, it was thus not possible heretodate to effect a complete decoupling of the combustion, heat transfer and environmental control processes therewith, and as a consequence of this fact, with such prior art forms of fluidized-bed boilers heretofore, there has not existed the possibility of allowing each of these processes, i.e., the combustion process, the heat transfer process and the environmental control process, to be separately optimized.
It is, therefore, an object of the present invention to provide a new and improved design for a heat transfer system that is predicated upon the employment therefor of a new and novel approach insofar as heat transfer systems are concerned.
It is another object of the present invention to provide such a new and improved heat transfer system that is characterized by its low cost.
It is still another object of the present invention to provide such a new and improved heat transfer system that is characterized by the fact that solids enhanced heat transfer is capable of being realized therewith.
It is yet another object of the present invention to provide such a new and improved heat transfer system that is characterized by the fact that therewith there is a complete decoupling of the combustion, heat transfer and environmental control processes.
Another object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that by virtue of the complete decoupling therewith of the combustion, heat transfer and environmental control processes, it thus enables each of these processes to be separately optimized.
Still another object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that the heat transfer solids, e.g., bauxite, are effectively separated from the solid fuel ash, sorbent, combustibles, and flue gas in a classification step before these heat transfer solids are caused to flow to a heat transfer means.
A still another object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that such a heat transfer system is not affected by changing fuel properties, be the fuel a solid, a liquid or a gas by virtue of the existence of the classification process employed therewith whereby only the heat transfer solids, e.g., bauxite, are in contact with the heat transfer means.
A yet another object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that to the extent that an internal heat source is employed in connection with such a new and improved heat transfer system there is thus no heat transfer surface embodied in the area of the internal heat source.
A further object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that such a heat transfer system nevertheless still retains the capability to effect therewith a minimization of NOx emissions.
Yet an object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that therewith sulfur capture is decoupled from the combustion process.
Yet a further object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that in accordance with the best mode embodiment thereof the need for a fluidized bed heat exchanger is eliminated therewith with the concomitant benefits being derived as a consequence thereof that auxiliary power is reduced and the cost of blowers and ductwork associated therewith is avoided, although it is still possible with such a new and improved heat transfer system to have a fluidized bed design wherein external heat transfer surface is followed by a counter current section at one end thereof.
Yet another object of the present invention is to provide such a new and improved heat transfer system that is characterized by the fact that it is possible therewith to employ a cold cyclone in lieu of a hot cyclone, the latter being what is customarily more generally required to be utilized.
Yet still another object of the present invention is to provide such a new and improved heat transfer system that is advantageously characterized in that such a heat transfer system is relatively inexpensive to provide, while also being relatively simple in construction.
In accordance with the present invention there is provided a new and improved heat transfer system, the design of which is predicated upon the employment therefor of a new and novel approach insofar as heat transfer systems are concerned. More specifically, the subject heat transfer system of the present invention represents a new and novel approach to designing a low cost heat transfer system using solids enhanced heat transfer. The concept, which the subject heat transfer system of the present invention embodies, involves a complete decoupling of the combustion, heat transfer and environmental control processes, thus allowing each to be separately optimized. Based on a cost comparison between the heat transfer system of the present invention and a 100 MW circulating fluidized bed system of conventional construction, it has been determined from the results of such a cost comparison that the costs of all pressure parts for the heat transfer system of the present invention could be reduced approximately 65% from that of the 100 MW circulating fluidized bed system of conventional construction, and that significant reductions in structural steel, plant footprint, and building volume are also achievable with the heat transfer system of the present invention versus that attainable with the 100 MW circulating fluidized bed system of conventional construction.
Continuing, the subject heat transfer system of the present invention employs a hybrid design capable of operating at high temperatures, e.g., up to 1100 degrees C., and with low solids recirculation rates from the cyclone. A second solids circulation loop is also superimposed thereupon. In accord with the mode of operation of the heat transfer system of the present invention, a dense stream of cold solids is introduced into the top of a first portion thereof. These solids are then heated as a result of a recuperative heat transfer, which occurs within this first portion of the heat transfer system of the present invention, between these cold solids and a heat source, which in turn may be generated either within this first portion of the heat transfer system of the present invention or externally thereof, as these solids drop towards the bottom of this first portion of the heat transfer system of the present invention, while the heat source itself in turn is being cooled down so as to be at a low temperature at the outlet of this first portion of the heat transfer system of the present invention. The hot bed solids are drained from this first portion of the heat transfer system of the present invention into a plenum heat exchanger which although not required in accordance with the best mode embodiment of the invention is located below this first portion of the heat transfer system of the present invention. In this regard, the plenum heat exchanger need not be located directly under the combustor so long as the plenum heat exchanger is located near enough to the combustor such that the heat transfer solids can flow downward by gravity from the combustor into the plenum heat exchanger. All of the heat transfer surface of the heat transfer system of the present invention, in accord with the best mode embodiment of the invention, is located in this plenum heat exchanger. In accord with the mode of operation of the heat transfer system of the present invention, the solids slowly move downward through this plenum heat exchanger in a manner, which in accord with the best mode embodiment of the present invention is similar in nature to that of a moving bed. The direct contact of the hot solids with the tubes, which are suitably located for this purpose within the plenum heat exchanger, provides a high rate of conductive heat transfer therebetween and reduces the total amount of heat transfer surface requirements.
Some of the key features that serve to advantageously characterize the heat transfer system of the present invention vis-a-vis prior art forms of heat transfer systems are the following: a) significantly reduced heat transfer surface, b) high temperature Rankine cycles are possible with the technology that the heat transfer system of the present invention embodies, c) simple pressure part design, d) standard pressure part design, e) simple support design, f) reduced gas side pressure drop, and g) process optimization. Significantly reduced heat transfer surface is achieved by virtue of the fact that in accordance with the design, which the heat transfer system of the present invention embodies, all pressure part heat transfer surface is consolidated into a single counterflow heat exchanger, which is located relative to the aforereferenced first portion of the heat transfer system of the present invention such that it is possible for the heat transfer solids to flow downwardly by gravity from the combustor to said heat exchanger. As such, the direct contact between the hot solids and the heat transfer surface provides high heat transfer rates for all surfaces. In addition, extended surfaces can be utilized in the heat transfer system of the present invention, which further reduces the requirements for heat transfer surface. A cost comparison study has shown that the total pressure part weight and cost for the heat transfer system of the present invention would be about one-third that of a circulating fluidized bed system operating at the same design conditions as the heat transfer system of the present invention.
The heat transfer system of the present invention enables high temperature Rankine cycles and their high plant efficiencies to be utilized without the need for developing or using exotic materials. Furthermore, the high heat transfer rates obtained through the use in the heat transfer system of the present invention of the moving bed-like movement of the hot solids moving bed, in accord with the best mode embodiment of the present invention, eliminates the need for very high temperature differentials between such hot solids and the tubes of the plenum heat exchanger and concomitantly reduces maximum tube metal temperatures. High temperature steam conditions can thus be realized with moderate temperatures within the aforereferenced first portion of the heat transfer system of the present invention thereby enabling the use of readily available high nickel alloys. Tests have shown that adding extended surface to the tubes of the plenum heat exchanger of the heat transfer system of the present invention has a dramatic impact on the heat transfer surface requirements. In this regard, the high heat transfer rates and extended tube surfaces, which are attainable with the heat transfer system of the present invention, greatly reduce the cost of all heat transfer sections, with about a 50% reduction being achievable in the expensive high temperature sections. If so desired, additional surface reductions are possible with the development of high temperature finned surfaces.
The heat transfer system of the present invention functions as a once through heat transfer system with a single circuit for economizer, evaporator and superheater. The single section superheater thereby eliminates the need for intermediate headers. Furthermore, where applicable, the heat transfer system-turbine connecting piping is greatly reduced because the steam outlets from the heat transfer system of the present invention are located at the same elevation as the turbine. With the heat transfer system of the present invention steam-side and gas-side imbalances can be minimized as a consequence of controlling solids flow over the different tube sections thereof. Moreover, there is no requirement for sootblowers, since the heat transfer sections do not come in contact with the fuel ash. Additionally, the conductive heat transfer, which is produced as a consequence of the moving bed-like movement, in accordance with the best mode embodiment of the present invention, provides a uniform heat flux around the tube centerline, unlike the waterwalls, which are commonly employed in prior art heat transfer systems, that are subjected to one-sided heating. What""s more, since the heat transfer system of the present invention lacks waterwalls, waterwall limitations due to a mix of austenitic/ferritic materials or stress differentials due to single sided heat fluxes, which serve to disadvantageously characterize prior art heat transfer systems, are eliminated. In addition, high temperature corrosion to which prior art heat transfer systems are known to be subjected, is also eliminated with the heat transfer system of the present invention.
As is well known to those skilled in the art, the pressure part arrangement for a circulating fluidized bed system of conventional construction must be designed for the specific fuels fired in the combustor thereof. It is also well known to those skilled in the art that the gas flow rate through the backpass of a circulating fluidized bed system of conventional construction increases with higher fuel moisture. Therefore, the tube spacing in the backpass of a circulating fluidized bed system of conventional construction must be increased for high moisture fuels to maintain proper gas velocities through such tubes, thus resulting in larger and more expensive backpasses in the case of circulating fluidized bed systems of conventional construction. Accordingly, insofar as circulating fluidized bed systems of conventional construction are concerned, the combustor thereof must be designed to accommodate the worst fuel when multiple fuels are required.
On the other hand, the heat transfer surface in the heat transfer system of the present invention is not affected by changing fuel properties, either when an internally generated heat source is employed in connection with the heat transfer system of the present invention or when an externally generated heat source is employed in connection therewith. This stems from the fact that in neither case do the combustion gases and fuel ash contact the heat transfer surface of the heat transfer system of the present invention. This is because of the inclusion of a classification process to which reference will be had hereinafter, which in accordance with the best mode embodiment of the present invention is located before the plenum heat exchanger, such that this classification process is operative to separate the heat transfer solids, e.g., bauxite, from the solid fuel ash, sorbent, combustibles and flue gas. In addition, the heat transfer system of the present invention will have higher gas velocities through the first portion thereof with high moisture fuels, when an internally generated heat source is employed in connection with the heat transfer system of the present invention. Finally, when an internally generated heat source is employed in connection with the heat transfer of the present invention, heat recuperation in the first portion of the heat transfer system of the present invention can be maintained for different fuels through changes in recirculating particle size and recirculation rate.
Continuing, the first portion of the heat transfer system of the present invention does not embody any heat transfer surface therewithin, and is thus ideal for a cylindrical, self-supporting design with a thin refractory shell. Moreover, such an arrangement, insofar as the heat transfer system of the present invention is concerned, eliminates the need for buckstays and greatly reduces structural steel requirements. In addition, since the heat source is cooled within the first portion of the heat transfer system of the present invention, the cold cyclone will be significantly smaller than that employed in circulating fluidized bed-systems of conventional construction and concomitantly will require only small amounts of refractory and structural steel. Also, with the heat transfer system of the present invention, the support requirements for the heat exchangers thereof are substantially reduced because the tube bundles employed in such heat exchangers are located close to the ground and are much lighter than those that are employed in a circulating fluidized bed system of conventional construction.
It is also to be noted that the solids circulation rate in the heat transfer system of the present invention is much less than that in a circulating fluidized bed system of conventional construction, and thus has a lower gas side pressure drop. Also, the heat exchanger through which the hot solids move in a moving bed-like fashion, in accordance with the best mode embodiment of the present invention, which is employed in the heat transfer system of the present invention, eliminates the need, in accordance with the best mode embodiment of the present invention, for a fluidized bed heat exchanger (FBHE), the latter being a component that is commonly employed in a circulating fluidized bed of conventional construction, which in turn reduces auxiliary power requirements and the cost of blowers and ductwork.
From the foregoing, it should be readily apparent that the heat transfer system of the present invention provides some unique opportunities for process optimization because with the heat transfer system of the present invention, the combustion, heat transfer, and environmental control processes are effectively decoupled. Yet, with the heat transfer system of the present invention conventional fluidized bed system fuel flexibility is still capable of being maintained within the high temperature first portion thereof, coupled with cyclone recycle for carbon burnout. In addition, the following features are also attainable with the heat transfer system of the present invention: NOx emissions can be minimized in the lower part of the first portion of the heat transfer system of the present invention; sulfur capture is decoupled from the heat source generating process of the heat transfer system of the present invention by utilizing a suitable backend system for this purpose; and limestone may still be calcined in the first portion of the heat transfer system of the present invention although a requirement thereof, in accordance with the best mode embodiment of the present invention, is that such limestone be fine enough to pass through the first portion of the heat transfer system of the present invention in a single pass. However, it is recognized that there may be situations such as for very high sulfur coals wherein it may be desirable to try and obtain some sulfur capture in the first portion of the heat transfer system. In such a situation, it might be desirable to size the limestone such that the limestone will be subjected to recirculation a few times before passing through a cyclone.